1) Field of the Invention
The present invention relates to a belt unit including a belt member and a plurality of belt stretching members that stretch and support the belt member to make an endless movement, an image forming apparatus including the belt unit, and a method of driving and controlling the belt member.
2) Description of the Related Art
An image forming apparatus that forms a multi-color toner image by superposing a plurality of single-color toner images on an endless belt member (surface of the belt or a paper carried thereon) is currently in use. The method of superposing single-color toner images on the belt member includes a transfer method and a direct recording method. The transfer method has two types: one employs one image carrier; and the other employs a plurality of image carriers.
An example of the image forming apparatus that superposes single-color toner images using the transfer method employing one image carrier is disclosed in Japanese Patent Application Laid Open No. 2002-108037. In this image forming apparatus, single-color toner images of different colors are formed on a photosensitive element using an electrophotographic process each time an intermediate transfer belt make one lap of movement, and these toner images are superposed on the intermediate transfer belt.
Japanese Patent Application Laid Open No. Hei 10-31373 discloses an image forming apparatus that superposes single-color toner images using the transfer method employing a plurality of image carriers. In this image forming apparatus, single-color toner images of different colors are formed respectively on a plurality of photosensitive elements that are arranged opposite to the intermediate transfer belt using the electrophotographic process, and these toner images are sequentially superposed on the intermediate transfer belt that makes an endless movement.
Japanese Patent Application Laid Open No. 2000-94374 discloses an image forming apparatus that superposes single-color toner images using a direct recording method. In this image forming apparatus, a toner image is formed without using the electrophotographic process in which an electrostatic latent image is developed with toner. More specifically, a plurality of image forming units are provided in the image forming apparatus, each of which is capable of directly recording the toner image on an intermediate recording belt by spraying a bunch of toner in the form of dots through a plurality of holes provided in an electronic board. The intermediate recording belt is endlessly moved along the positions facing these image forming units. The single-color toner images of different colors are sequentially superposed on the intermediate recording belt, and a color image is recorded thereon.
In all of the methods, the speed in the endless movement of the belt member is changed by deviation in thickness of the belt member in the circumferential direction, a decentered axis of a belt drive motor or a gear, a pitch error of the gear, an engagement error of the gear, etc. If the speed of the belt member is changed, the toner images are misaligned on the belt member as a result of inaccurate superposition. The misalignment due to the inaccurate superposition of the toner images results in irregularities in color of a multi-color toner image. The purpose of superposition of toner images is to obtain a multi-color toner image. However, a multi-color toner image may be obtained by superposing toner images of the same color for some reason. In this case, the misalignment of the toner images causes the shape of the image to be largely deformed. Further, even if a monochrome image is to be formed, the quality of the image finally formed on a paper may be degraded by the change in the speed during transfer of the image.
On the other hand, Japanese Patent Application Laid Open No. Hei 11-24507 discloses a technology of using an intermediate recording belt with a scale having a plurality of scale marks at predetermined pitches in the circumferential direction of the belt member. In this technology, the speed of the intermediate recording belt is controlled based on a result of reading the scale marks to minimize the change in belt speed. By employing such a configuration, it is possible to minimize the misalignment of the toner images in superposition due to the change in the belt speed.
However, it is extremely difficult to precisely adjust pitches each between scale marks (hereinafter, “scale pitch”) of the scale provided on the belt member to each other. For example, in the image forming apparatus disclosed in Japanese Patent Application Laid Open No. Hei 11-24507, a scale seal made of a plastic sheet on which scale marks are formed by a laser or the like, is adhered to the periphery of the belt member. If the scale seal is adhered to the belt member in such a manner that small wrinkles occur, the scale pitch at a wrinkled portion goes out of adjustment.
Furthermore, there is no way to prevent occurrence of any circumferential error in the belt member. If the circumferential error occurs, the front end and the rear end of the scale seal, which is wound along the belt member, are not perfectly joined to each other. Therefore, both of the ends may be overlapped or largely separated from each other, and a scale pitch goes out of adjustment at a joint.
There is also a method of directly writing scale marks on the surface of the belt member by printing or laser machining without using the scale seal. However, if the circumferential error occurs, a pitch between an initially written scale mark and a finally written scale mark goes out of adjustment by no means. Further, there is another method of forming linear toner images at a specific pitch along the edge of the belt member in its lateral direction each time an image is formed, and using the toner images as scale marks. However, as explained above, the obtained pitch between an initially written linear toner image and a finally written linear toner image is inevitably different from the others.
As explained above, even if the scale marks are calibrated on the belt member by using any of the methods, the difference in the pitch between the scale marks is not avoided if an error occurs in the circumference of the belt member.
If any abnormality occurs in the scale pitch due to the above-mentioned conditions, a detected scale pitch is changed due to a part where the abnormality occurs (hereinafter, “abnormal part”), and this change is incorrectly detected as if this change is caused by the change in speed of the belt member. Therefore, the drive control of the belt reflects the result of the incorrect detection. Consequently, although the belt is actually driven at a constant speed, the speed may possibly be changed, which may in turn foster the change in the speed.